Convert SVG images to PNG

RTFD generates PDF documentation, but it lacks images since
they are in incompatible format. This patch adds tox target
svg2png that converts images and replaces references to
wild-card.

Change-Id: I0672328fb8012d34dc91867aa49f19cfbd19890d

tools/svg2png.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+for f in doc/source/images/*svg; do cairosvg ${f} -o ${f/\.svg/\.png} ; done